Washington harbor seal stable isotope data

Understanding the response of predators to ecological change at multiple
temporal scales can elucidate critical predator-prey dynamics that would
otherwise go unrecognized. We performed compound-specific nitrogen stable
isotope analysis (CSIA) of amino acids on 153 harbor seal museum skull
specimens to determine how trophic position of this marine predator has
responded to ecosystem change over the past century. The relationships
between harbor seal trophic position, ocean condition, and prey abundance,
were analyzed using hierarchical modelling of a multi-amino acid framework
and applying 1-, 2-, and 3- year temporal lags. We identified delayed
responses of harbor seal trophic position to both physical ocean
conditions (upwelling, sea surface temperature, freshwater discharge) and
prey availability (Pacific hake, Pacific herring and Chinook salmon).
However, the magnitude and direction of the trophic position response to
ecological changes depended on the temporal delay. For example, harbor
seal trophic position was negatively associated with summer upwelling, but
had a 1- year delayed response to summer sea surface temperature,
indicating some predator responses to ecosystem change are not immediately
observable. These results highlight the importance of considering dynamic
responses of predators to their environment as multiple ecological factors
are often changing simultaneously and can take years to propagate up the
food web.